The invention relates to a high-pressure discharge lamp which is provided with a discharge vessel that encloses a discharge space, has a ceramic wall and is closed by a ceramic plug, said discharge space accommodating an electrode which is connected to an electric current conductor by means of a leadthrough element which projects into the ceramic plug with a tight fit, is connected thereto in a gastight manner by means of a sealing ceramic and has a first part which forms a cermet at the area of the gastight connection.
A lamp of the kind set forth is known from U.S. Pat. No. 5,424,609 (=EP 0587238). The filling of the known lamp contains metal halide in addition to mercury.
In the context of the present description and the claims the term xe2x80x9cceramic wallxe2x80x9d is to be understood to mean a wall of metal oxide, for example sapphire, sintered polycrystalline Al2O3 or YAG, as well as a wall of metal nitride, for example AIN.
The known lamp has a comparatively low power of 150 W at the most at an arc voltage of approximately 90 V. Because the electrode in such a lamp conducts comparatively small currents during operation of the lamp, the dimensions of the electrode may remain comparatively small so that a comparatively small internal diameter of the projecting plug suffices. In the case of a lamp having a rated power in excess of 150 W, or a substantially lower arc voltage, so in the case of large electrode currents, electrodes of larger dimensions are required. Consequently, the internal plug diameter will be larger accordingly. It has been found that in such lamps there is an increased risk of premature failure, for example due to breaking off of the electrode or cracking of the plug.
It is an object of the present invention to provide a way to mitigate said drawbacks.
To this end, a high-pressure discharge lamp of the kind set forth is characterized accordance with the invention in that the leadthrough element also includes a second part which is a metal part and extends from the cermet in the direction of the electrode.
It is an advantage of the lamp in accordance with the invention that surprisingly it has been found that an internal plug diameter of more than one millimeter can be used in the case of lamps that are suitable for larger electrode currents, and that premature failure of the lamp due to breaking off of the electrode or cracking of the plug is effectively counteracted. As a result of this construction of the leadthrough element the first part of the leadthrough element can be optimized in respect of the coefficient of expansion relative to the ceramic plug. To this end, the cermet preferably has a metal content of at the most 45% by volume, but preferably no more than 35% by volume. This is beneficial to the realization of a gastight connection that is capable of withstanding thermal shocks to a high degree. Because of the presence of the second, metal part of the leadthrough element the cermet is exposed to less high temperatures during operation of the lamp. Processes of attack, if any, will thus be delayed; this has a favorable effect on the service life of the lamp. The cermet is a sintered composition of a ceramic material and a metal. The metal of the cermet preferably corresponds to that of the metal part of the leadthrough element. This is beneficial to the realization of a solid connection between the cermet and the metal part of the leadthrough element. Metals that are suitable in this respect are preferably Mo and W, because each of these metals has a very high melting point and is capable of withstanding halogenide to a significant degree.